The present invention relates generally to operation of compression ignition engines that may be used in various machines, such as locomotives, marine vessels, etc., and, more particularly the present invention is related to controlling engine operation during a bog condition.
Large self-propelled traction vehicles such as locomotives commonly use a diesel engine to drive an electrical system comprising generating means for supplying electric current to a plurality of traction motors, such as alternating current or direct current (dc) motors whose rotors are drivingly coupled to the respective axle-wheel sets of the vehicle. The generating means typically comprises a main 3-phase traction alternator whose rotor is mechanically coupled to the output shaft of the engine (typically a 16-cylinder turbocharged diesel engine). When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase stator windings of the alternator. These voltages are rectified and, in the case of DC traction motors, applied to the armature windings of the traction motors, or, in the case of AC traction motors, applied to inverters for suitable variable frequency conversion prior to being applied to energize such AC motors.
During the "motoring" or propulsion mode of operation, a locomotive diesel engine tends to deliver constant power, depending on throttle setting and ambient conditions, regardless of locomotive speed. For maximum performance, the electrical power output of the traction alternator must be suitably controlled so that the locomotive utilizes full engine power. For proper train handling, intermediate power output levels are provided to permit graduation from minimum to full output. But the load on the engine must not exceed whatever level of power the engine can develop. Overloads can cause premature wear, engine stalling or "bogging," or other undesirable effects. Historically, locomotive control systems have been designed so that the operator can select the desired level of traction power, in discrete steps between zero and a maximum level, so that the engine develops whatever level of power the traction and auxiliary loads demand.
Engine horsepower is proportional to the product of the angular velocity at which the crankshaft turns and the torque opposing such motion. For the purpose of varying and regulating the amount of available power, it is common practice to equip a locomotive engine with a speed regulating governor which adjusts the quantity of pressurized diesel fuel (i.e., fuel oil) injected into each of the engine cylinders so that the actual speed (RPM) of the crankshaft corresponds to a desired speed. The desired speed is set, within permissible limits, by a manually operated lever or handle of a throttle that can be selectively moved in eight steps or "notches" between a low power position (N1) and a maximum power position (N8). The throttle handle is part of the control console located in the operator's cab of the locomotive. In addition to the eight conventional power notches, the handle has an "idle" position.
The position of the throttle handle determines the engine speed setting of the associated governor. In a typical electronic fuel injection governor system, the output excitation from a controller drives individual fuel injection pumps for each cylinder allowing the controller to individually control start of and duration of fuel injection for each cylinder. The governor compares the commanded speed (as commanded by the throttle) with the actual speed of the engine, and it outputs signals to the controller to set fuel injection timing to minimize any deviation therebetween.
For each of its eight different speed settings, the engine is capable of developing a corresponding constant amount of horsepower (assuming maximum output torque). When the throttle notch 8 is selected, maximum speed (e.g., 1,050 rpm) and maximum rated gross horsepower (hp) (e.g., 4,000 hp) are realized. Under normal conditions, the engine power at each notch equals the power demanded by the electric propulsion system which is supplied by the engine-driven main alternator plus power consumed by certain electrically and mechanically driven auxiliary equipment.
The output power (KVA) of the main alternator is proportional to the product of the rms. magnitudes of generated voltage and load current. The voltage magnitude varies with the rotational speed of the engine, and it is also a function of the magnitude of excitation current in the alternator field windings. For the purpose of accurately controlling and regulating the amount of power supplied to the electric load circuit, it is common practice to adjust the field strength of the traction alternator to compensate for load changes (traction motor loading and/or auxiliary loading) and minimize the error between actual and desired KVA. The desired power depends on the specific speed setting of the engine. Such excitation control will establish a balanced steady-state condition which results in a substantially constant, optimum electrical power output for each position of the throttle handle.
As suggested above, under normal operating conditions, the RPM of the engine is closely regulated. Unfortunately, under certain anomalous conditions, such as may occur due to a temporary overload or failure of a speed sensor that senses actual engine RPM, the engine could loose its speed regulation, that is, engine RPM would drop far below commanded engine RPM. The above-described condition is generally referred to as a bog. In a bog condition, the drop in engine RPM results in a large engine RPM error that demands a maximum amount of available fuel. In the case of a controller that uses a proportional plus integral (PI) control loop, the gain of the integrator during the bog condition may wind up to a maximum value so that, when the cause of the bog is removed, the engine, due to excess fuel and/or high gain in its control loop, may accelerate very rapidly (e.g., in excess of 100 RPM/second), and such high acceleration results in undesirable stresses and wear on various rotating components of the locomotive, e.g., cycle skippers, fans, etc. Further, the high acceleration may cause a drastic disturbance on the electrical system of the locomotive. That disturbance results in undesirable electrical transients on the various components and control panels interconnected to the electrical system. Additionally, the high acceleration could result in engine speed overshoot on recovery.
Thus, it would be desirable to provide a control system and method that would allow for a graceful recovery of engine speed and load during and subsequent to a bog condition. It would be further desirable to reduce the magnitude of transients that various components of the locomotive have generally encountered during bog conditions.